On 6/7/2013 4:51 PM, Tom Ellis wrote:
On Fri, Jun 07, 2013 at 04:05:09PM -0400, Joe Q wrote:
The phantom parameter solves the same problem as scoped type variables.
Granted, if you find yourself in that kind of polymorphic soup you have
deeper problems...
I don't understand this. Scoped type variables are used when you want to
use a type variable from the top level within the body of a function. If
you use "op" and specify a particular constructor then you don't have a
variable but a concrete instance of a type. But maybe I'm missing some more
powerful way this can be used ...
Tom
You can use scoped type variables to correct an ambiguous type error.
You can think of op as a variation on asTypeOf, as documented here on
http://www.haskell.org/haskellwiki/Scoped_type_variables#Avoiding_Scoped_Type_Variables.
If I tried to come up with an example that's specific to op, it would
only be horribly contrived.
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
